Many electronic applications benefit from the ability to multiplex several time varying electrical signals at different frequencies and possibly within different frequency bands into a single multi-frequency composite signal, and conversely, to demultiplex a multi-frequency electrical signal into its constituent individual electrical signals, each at a different frequency, and possibly in different frequency bands. For example, many communication systems, signal analysis devices, and other electronic devices discriminate, or filter, a signal in a frequency band of interest from a signal that may contain signals at many different frequencies.
An example of a two-way frequency multiplexer is a diplexer located in a communication device. A diplexer generally comprises a pair of band pass filter circuits coupled between an antenna (a common port) and transmits and receives circuitry in the communication device. A diplexer generally includes a first band pass filter in the form of a transmit band pass filter and a second band pass filter in the form of a receive band pass filter. The transmit band pass filter passes signals in a transmit frequency band and the receive band pass filter passes signals in a receive frequency band, which is different than the transmit frequency band. One side of the transmit band pass filter and the receive band pass filter is coupled to the antenna and the other side of each filter is coupled to the transmit and receive circuitry, respectively.
The two band pass filters allow full duplex communication by allowing transmit and receive signals to be processed simultaneously. The transmit band pass filter is designed to allow a transmit signal to pass, while not significantly attenuating the strength of the receive signal. The receive band pass filter is designed to pass a receive signal, while not significantly attenuating the strength of the transmit signal. Because the transmit signal is generally significantly higher in signal strength than the receive signal, the receive filter is also designed to attenuate the transmit signal to a level that will not interfere with the generally low-level receive signal and the sensitive receive circuitry.
Multiplexers of order n (where n is larger than 2) can be built to operate in a similar way, where a given frequency band will pass through one filter and be unaffected by the other n−1 filters. The conventional configuration for these multiplexers includes n filters connected in parallel at a common port. In order for the inactive filters to not load down this common port, they should be constructed to have input impedance much greater than the system impedance for frequencies in their stop bands. The stop band is the frequency range within which a filter is not resonant and will no longer pass the signal.
Unfortunately, this configuration is incompatible with many useful resonator circuits, including but not limited to, quartz crystal filters and film bulk acoustic resonator (FBAR) filters, because such resonators have an intrinsic shunt capacitance that makes them incapable of having a high input impedance in their stop bands.
If any significant number of these resonant circuits are combined in parallel at the common port of a frequency multiplexer, the impedance resulting from the combined capacitance of the resonators tends to fall below the system impedance, and thus degrades the performance of the multiplexer below the point of usefulness.
Therefore, it would be desirable to have a frequency multiplexer that exhibits acceptable performance using available resonator circuits.